A problem with mortars used on different armored self-propelled chassis concerns the ability of the device to defend itself. If a heavy mortar is mounted on a chassis, it is not usually easy to mount on it sufficiently heavy artillery weapons for defense purposes. Lighter automatic weapons, on the other hand, are not always sufficient against antitank equipment or other artillery at a longer distance. In some cases it would be necessary to have the possibility to fire a smooth-bore mortar also horizontally with direct fire, which is not normally possible. Any movement of the shell in the barrel can also cause the shell not to go off when needed. One solution to this problem is proposed in the U.S. Pat. No. 5,503,080, in which a separate guiding and attachment element is used in the tail of a mortar shell. Said solution includes a separate guiding element furnished with grooves dimensioned according to the tail of the shell, the tail being pushed into the grooves in such a way it is attached to the guiding element with a friction joint. A problem with this solution is that the fastening of the support piece is not reliable during handling, the operational reliability of the shell being thus insufficient. The application of said solution also requires very precise measures of the tail of the shell and its support piece, in order for the shell to function at least somewhat reliably.